The present invention relates to a fuel injector.
German Patent No. 195 19 192 describes a fuel injector that has a piezoelectric actuator, a valve needle operated by the actuator by a hydraulic step-up mechanism of the lift and a valve closing body operated by the valve needle and cooperating with a valve seat face to form a sealing seat. The stroke step-up mechanism here also assumes the function of temperature compensation, for which purpose annular gaps are provided in the stroke step-up mechanism, permitting fuel to flow into and out of a stroke step-up space for compensation of a temperature-induced change in length of components of the fuel injector.
The fuel injector known from German Patent No. 195 19 192 has the disadvantage that fuel flows into and out of the stroke step-up space through the annular gap only at a low fuel flow, so the time required to completely fill the stroke step-up space is much longer than one injection cycle. The known fuel injector is therefore not suitable for volatile fuels such as gasoline. When the fuel used is gasoline, some gasoline evaporates out of the stroke step-up space when the fuel injector does not operate for a long time, in particular at high ambient temperatures, so that when the actuator is operated, there cannot be a dynamic transfer of force to the valve needle and thus the fuel injector cannot be operated quickly enough.
German Patent No. 37 42 241 describes a fuel injector having a hydraulic stroke step-up device, in which hydraulic oil is used as the hydraulic medium. Temperature compensation here takes place through a non-return valve arranged in a valve needle and through annular gaps provided between the valve needle and a cylinder or between the cylinder and a sleeve. In operation of a fuel injector, the cylinder displaces a portion of the hydraulic oil out of the stroke step-up space, thus operating the valve needle against the direction of displacement of the cylinder.
The fuel injector according to the present invention has the advantage over the related art that a temperature compensation space is formed by a simple design measure, utilizing the fuel pressure to fill the temperature compensation space completely with fuel through a directional valve within a short period of time.
It is advantageous that the directional valve is connected to the interior of the needle driving sleeve, and it is especially advantageous that the needle driving sleeve has a lateral recess which is part of a fuel line connecting the fuel inlet to the temperature compensation space over the directional valve. This permits a compact design of the temperature compensation device.
In an advantageous manner, a closing spring provided in an interior space of the needle driving sleeve; this closing spring acts on the fuel pressure in the temperature compensation space by way of the valve needle shoulder. Space provided in the interior of the needle driving sleeve can be utilized to accommodate the closing spring, thus yielding a more compact design of the fuel injector.
It is advantageous that the valve needle has a recess and that a ball arranged in the recess to form the directional valve cooperates with a concentric sealing face, in particular a conical sealing face of the recess, to form a sealing seat. Integrating the directional valve into the valve needle makes it possible to save on parts, and manufacturing complexity can be reduced due to the simple design.
An adjusting pin is advantageously provided in the recess to adjust a maximum opening motion of the ball and is connected to the valve needle. In particular, the response characteristic of the directional valve can be predetermined by adjusting the maximum opening motion.
It is advantageous that an annular gap provided between the valve needle shoulder and the needle driving sleeve permits fuel to escape from the temperature compensation space in a temperature-induced change in length of parts of the fuel injector. The guide face provided between the valve needle shoulder and the needle driving sleeve is designed in this way so that it also permits venting of the temperature compensation space. It is also advantageous that an annular gap is provided between the valve needle and the shoulder of the needle driving sleeve.